In OWA, as happens with the Office 365 home page, the entire SharePoint Online and OneDrive For Business (ODFB) services, search is powered by Microsoft Searc. As soon as we make a click in the Search Box in OWA, Microsoft Search starts to suggest search results to the user:
Moreover, Microsoft Search in OWA provide a set of default search filters that can be applied to e-mail items:
And once we click the “Search” button, we get in OWA the results matching the search filters defined by the user: